1. Technical Field of the Invention
This invention relates generally to electromagnetic transducer motor structures, and more specifically to the material composition of the yoke and other steel parts in the magnetic circuit of an induction motor for a transducer such as a loudspeaker or a microphone.
2. Background Art
Electromagnetic transducers utilize a variety of different types of motors. The most common of these is the moving coil motor, in which a magnetic circuit provides magnetic flux over a magnetic air gap, and an alternating current voice signal is applied to a multi-winding voice coil suspended in the magnetic air gap; the alternating current voice coil signal generates an oscillating magnetic field which interacts with the magnetic circuit's flux in the air gap, causing the voice coil to oscillate axially within the air gap, in turn driving the diaphragm assembly and generating acoustic waves corresponding to the voice signal.
A much less common type of motor is the induction motor, in which a magnetic circuit provides magnetic flux over a magnetic air gap, and an alternating current voice signal is applied to a stationary multi-winding primary coil disposed somewhere in the magnetic circuit; the alternating current voice signal causes the magnetic flux in the air gap to oscillate, which induces an alternating current in a “shorted turn” single-turn coil disposed in the magnetic air gap. The alternating current in the shorted turn generates an oscillating magnetic field, which interacts with the magnetic circuit's flux in the air gap, causing the shorted turn to oscillate axially within the air gap, in turn driving the diaphragm assembly to generate acoustic waves corresponding to the voice signal.
The induction motor is, in some sense, akin to an electrical transformer, in that it has a primary coil which is inductively coupled to a secondary coil (the shorted turn), and a ferrous yoke that supports the primary coil (and the secondary coil in the case of a transformer).
An early induction motor was taught in U.S. Pat. No. 2,621,261 to Karlsson et al., who discovered that “the [moving] coil may constitute one of the windings of a transformer, the iron circuit of which wholly or partly consists of the magnetic circuit.” Karlsson used “one short-circuited strip of copper” as his shorted turn, moving secondary coil. Karlsson further taught that “in order to reduce the losses of the iron circuit of the transformer, the [pole piece, cap, and cup yoke] are formed from a so called free cutting steel, which has been treated in a suitable way.” Free cutting steel (FCS) is steel which includes additives such as sulfur, lead, or calcium, to improve its machinability (see http://global.kyocera.com/prdct/tool/faq/index.html or http://www.sumitomometals.co.jp/e/news/news200-02-25.html). Karlsson used a free cutting steel cup housing a free cutting steel polepiece, a primary coil, and a radially charged magnet atop the primary coil and defining the magnetic air gap. Karlsson also used a perforated cap of free cutting steel which partially closes the magnetic circuit. Curiously, Karlsson placed his diaphragm inside the magnetic circuit, beneath the perforated cap (hence the perforations, to allow sound to escape).
More recently, Sony Corp. has been developing induction motor speakers. U.S. Pat. No. 5,062,140 to Inanaga et al. teaches an induction motor loudspeaker in which “the diaphragm is formed into a dome shape and comprises: a vibrating portion which is thinly formed into a semi-spherical shape; and a secondary coil constituted by a conductive portion which is thickly annularly formed at an opening edge portion. The whole diaphragm is a good conductor constructed of metal . . . ” Inanaga's induction motor uses an external magnet geometry, with a poleplate (or “T-yoke”), an axially charged ring magnet, and an annular top plate atop the magnet. Inanaga's primary coil is disposed at an inner diameter of the top plate, and forms the magnetic air gap with the polepiece. His conductive dome diaphragm has a cylindrical lower portion which constitutes the shorted turn secondary coil. Inanaga's innovation is a set of techniques for limiting induction of current in the domed remainder of the diaphragm to restrict the induced current to the shorted turn.
U.S. Pat. No. 6,359,996 to Ohashi, also assigned to Sony, teaches a variety of induction motor loudspeaker configurations. Some have internal magnet geometries, and some have external magnet geometries. In each configuration, the primary coil is disposed within the magnetic air gap, either on the inner surface of the magnetic air gap, or on both the inner and outer surfaces of the magnetic air gap and connected in series. Ohashi's innovation was to wind the primary coil(s) on its(their) own bobbin-like cylinder and to provide a step or groove in the back plate for positively positioning the cylinder(s) and primary coil(s), rather than e.g. winding the primary coil(s) directly on the top plate, cup, or polepiece.
In all those prior art induction motors, the induction motor drives the primary (and only) diaphragm; a non-moving primary coil drives a moving shorted turn which is rigidly coupled to or integral with the single diaphragm.
A few other inventors have developed coaxial, dual-diaphragm loudspeakers in which the center tweeter is inductively driven by the moving voice coil of the outer woofer.
U.S. Pat. No. 4,965,839 to Elieli teaches a coaxial loudspeaker in which the moving voice coil of the conventional, outer loudspeaker serves as a primary coil inductively driving a cylindrical skirt of a metallic tweeter dome. Elieli's innovation was to add a phase plug which appears to turn the inductively driven center tweeter into a compression driver.
U.S. Pat. No. 5,742,696 to Walton has teachings similar to Elieli's.
U.S. Pat. No. 6,542,617 to Fujihira et al., also assigned to Sony, is a curious example of a coaxial induction motor loudspeaker, in that there is only a single diaphragm which is coaxially driven. In low frequencies, the diaphragm is driven by a conventional moving voice coil motor. But in high frequencies, the diaphragm is driven by the electrically conductive bobbin which functions as a shorted turn. In the high frequencies, the moving voice coil mechanically separates from the bobbin by softening, liquefaction, or other such lowering of the bonding strength of the bonding agent used to affix the voice coil to the bobbin. The bonding agent functions, in essence, as a high pass filter, enabling the moving voice coil to act as a primary coil.
ATC Loudspeaker Technology Ltd of Gloucestershire, England, offers a line of loudspeakers whose drivers use a conventional moving voice coil motor. ATC's website offers a white paper (http://www.atc.gb.net/technology/Super_Linear_Technical.zip) discussing various benefits obtained by the addition of “Super Linear Magnetic Material” (S.L.M.M.) rings “which replace the steel regions concentric with the voice coil. ATC does not identify this material, but indicates that it offers high magnetic permeability and saturation level and low electrical conductivity. ATC indicates that the presence of these rings “increases the self-inductance of the voice coil. When eddy currents are allowed to circulate in the system, the oppose the magnetic field producing them (i.e. that from the coil) and ‘cancel out’ much of the self-inductance.”
An unnamed author writing for the audio recording magazine Sound On Sound alleges (http://www.soundonsound.com/sos/1997_articles/oct97/atcscm20a.html) alleges that these rings are “made from pressure-formed powdered iron to form part of the driver pole-piece. Using these rings to form the inner and outer surfaces of the magnetic air gap greatly reduces eddy currents in the pole pieces, producing a dramatic drop in the level of third-harmonic distortion—a problem that's plagued speaker designers ever since someone first had the bright idea of gluing a coil of wire onto the back of a cardboard cone.”
One significant drawback that has prevented induction motors from being more commonly used in electromagnetic transducers is that their steel structures, whose main function is to provide a low reluctance path for steering the magnetic flux to and from the magnetic air gap, are also electrically conductive. The oscillating magnetic fields which induce a desired alternating current in the shorted turn, and indeed the oscillating magnetic field generated by the alternating current in the shorted turn itself, also induce unwanted alternating “eddy currents” in any nearby, electrically-conductive parts. These induced currents have several significant, undesirable effects: they cause heating of those parts, they cause flux modulation, and they rob power that could otherwise be put to use driving the diaphragm.
What is needed, then, is an improved induction motor in which the susceptibility to unwanted, induced currents is reduced, minimized, or even eliminated. It appears that, until the present invention, the industry has not understood that improvements in the materials themselves of the magnetic circuit's steel components might be a way to make such improvements.